Direct Analysis of Photoresist by ICP-MS Featuring the Agilent Technologies 7500s ICP-MS 1
Presentation Outline How is photoresist used? Analytical challenges Instrumentation developments Analytical approach Tuning Calibration Typical analytical figures of merit Detection Limits (DLs) Spike recoveries at the 1 ppb level Stablitity Conclusions 2
What is Photoresist (PR)? Thick resin mixtures Two basic types: Positive resist: soluble upon exposure to radiation offer higher resolution than negative resists Negative resist: insoluble upon exposure to radiation 3
Process Steps Diffusion a layer of material such as an oxide layer is grown or deposited on to the wafer surface Next a positive photoresist layer is applied and cured on the oxide layer A glass mask is positioned over the wafer. Ultraviolet light shines through the clear portions of the mask and exposes the template onto the photo sensitive resist The exposed resist becomes soluble to the developer TMAH (2.38%) in the presence of UV light The mask pattern is then etched on to the wafer using either a wet or dry etching process The undeveloped/hardened photoresist is then removed and the process repeated using a different mask pattern 4
Trace Metal Requirements for Photoresist (PR) PR manufacturer and user requirements control impurities that affect characteristics of semiconductor devices limits on impurities are being reduced Typical acceptable levels of metallic impurities in the photoresist are in the range 10 30 ppb per element Typical elements of interest: Na, Mg, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn 5
Impurity Levels in Photoresist: Manufacturers Data Manufacturer T F S(1) S(2) Resin (wt. %) 30 28 28 30 Element Concentration (ppb) Na 10 11 29 247 Mg <10 1 - <20 K <10 22-20 Ca - 11-88 Mn <10 <1 - <20 Fe 30 - - 47 6 PR samples are typically analysed at 1:10 dilution in an appropriate solvent giving 2-3% resin in the sample as analyzed
Analytical Challenges (1) Some sample preparation is required prior to analysis of photoresist In the past acid digestion was widely used but it is time-consuming and leads to loss of volatiles - eg B, As contamination from apparatus, acid and other reagents potentially hazardous reactions More typically photoresist is diluted using an appropriate solvent e.g. 1:10 in N-methyl pyrolidone (NMP), propylene glycol mono-methyl acetate (PGMEA), ethyl lactate Detection limits in the photoresist are limited by impurity level in the solvents 7
Metal Impurities in Organic Solvents Impurity (ppb) Element Mode NMP Ethyl Lactate PGMEA Li (7) 0.013 0.044 0.009 Be (9) 0.099 0.080 0.027 B (10) - 17.690 1.296 Na (23) 8.201-0.051 Mg (24) 0.885 0.213 0.020 Al (27) 0.490 0.073 0.100 K (39) 0.459 2.870 0.039 Ca (40) 0.761 21.580 0.110 Ti (47) 1.021 0.887 2.010 V (51) 0.057 0.051 0.130 Cr (52) 0.857 0.086 2.035 Mn (55) 0.534 0.081 0.017 Fe (56) 1.631 0.683 1.147 8
Metal Impurities in Organic Solvents (2) Impurity (ppb) Element Mode NMP Ethyl Lactate PGMEA Ni (58) 1.024 0.072 0.064 Co (59) 0.031 0.043 0.022 Ni (60) 0.455 0.080 0.059 Cu (63) 1.855 0.306 0.013 Zn (68) 6.918 0.558 0.348 Mo (95) 0.062 0.026 0.054 Ag (!07) 0.109 0.022 0.019 Cd (111) 0.863 0.053 0.053 Ba (138) 0.037 0.034 0.021 Ta (181) 0.016 0.044 0.033 W (182) 0.027 0.013 0.029 Pb (208) 0.016 0.052 0.013 9
Analytical Challenges (2) Since photoresist is 30% resin, clogging of the nebulizer, torch, interface and drain can be a problem Heavy sample matrix could suppress analyte signals Carbon based spectral interferences on Mg (C 2 ) and Cr (ArC) 10
The Agilent Approach to Photoresist Analysis Clogging at nebulizer, torch, interface and drain Specially designed sample introduction system Low sample uptake rate PR spray chamber drain fitting Quartz nebulizer and torch injector with tapered tip Improved instrument design for analysis of high matrix samples Solid state ICP RF Generator replacement power tubes not needed Improved plasma stability Robust, 27.12 MHz high temperature plasma Carbon-based spectral interferences ShieldTorch cool plasma analysis at high power 950W power can handle heavy matrices such as PR 11
Tapered-tip Torch A Quartz narrow-bore injector (1.5 mm ID) with a tapered tip is recommended for PR analysis in place of standard 2.5mm ID torch A tapered tip avoids a point for deposition 12
Reducing Carbon-based Interferences Enhanced ShieldTorch System Long life shield plate Self aligning shield mount plasma AutoTune The combination of the ShieldTorch System and cool plasma conditions give effective interference removal of carbon based interferences on Mg (C 2 ) and Cr (ArC) Plasma Torch Load coil Shield plate 13
Experimental Standard Analytical Method for PR Analysis Define the analytical method Fix optional gas flow rate Tune cool plasma operating conditions Tune normal plasma operating conditions Sample preparation simple dilution Calibration using Method of Standard Addition 14
7500s Tuning for Photoresist Analysis General parameters 15 Fix sampling depth at 17.5mm Add 20% oxygen (0.2 L/min) to the plasma Free aspiration using 0.3mm capillary tubing Tune plasma conditions Optimize 59 Co sensitivity (10ppb aqueous solution) Verify reduction of Ar 2 dimer at mass 80 Verify reduction of 12 C (carbon does not ionize in cool plasma) Tune plasma conditions Optimize sensitivity for Li, Y and Tl with sampling depth and oxygen flow rate fixed Aspirate solution of PGME for approx. 10 minutes for change over to organic sample introduction
Analysis of Photoresist Dilute resist samples 1:10 with propylene glycol mono-methyl acetate (PGMEA) Prepare calibration standards (MSA) by spiking the diluted sample with 1, 2, 3 ppb multi-element standard Analyze A rinse step (using PGMEA) between samples is recommended to prevent signal drift resulting from PR deposition Following this protocol trades off sensitivity for simplicity. However, there are no gains to be had for this analysis by optimizing the ICP- MS for best possible performance 16
Method of Standard Addition (MSA) Calibration Measured signal and so reported concentration is sum of 1) background, 2) interference and 3) contamination. Instrument optimisation is used to minimise the contribution of 1) and 2) Contamination Residual Interference Random Background MSA calibration on one sample can be applied to all samples of the same matrix type, so analysis time is not compromised External calibration (with or without blank subtraction) can give under-reporting (reported value lower than true value), due to subtraction of too high a blank signal or signal changes due to sample transport and nebulization 17
7500s Operating Parameters ICP-MS Torch Nebulizer Agilent 7500s ShieldTorch System Quartz, 1.5 mm tapered injector Concentric Nebulizer (Self aspiration) RF Power Sampling depth Carrier gas flow rate Make up gas flow rate Optional gas flow rate Spray chamber temp Integration time / mass Plasma 1450W 17.5 1.0 L/min 0 L/min 20% 2 deg C 0.99 sec Plasma 950W 17.5 1.0 L/min 0.46 L/min 20% 2 deg C 0.99 sec 18 plasma conditions are achieved - even at 950W forward power. This ensures the plasma has enough energy to break down the sample matrix
Analysis of Photoresist Sample Element Mode Detection *Photoresist PGMEA A - B Limit (ppb) (A) (ppb) (B) (ppb) Li (7) 0.0004 0.001 0.000 0.001 Be (9) 0.002 0.003 0.006 B (10) 0.08 0.909 0.873 0.036 Na (23) 0.01 0.419 0.050 0.369 Mg (24) 0.005 0.119 0.015 0.105 Al (27) 0.06 0.113 0.103 0.010 K (39) 0.009 0.277 0.044 0.233 Ca (40) 0.02 0.288 0.162 0.127 Ti (47) 0.2 1.777 2.060 V (51) 0.057 1.983 2.024 Cr (52) 0.057 1.983 2.024 Mn (55) 0.006 0.009 0.003 0.006 Fe (56) 0.3 1.618 1.377 0.241 19 = not detected
Analysis of Photoresist Sample (2) Element Mode Detection *Photoresist PGMEA A - B Limit (ppb) (A) (ppb) (B) (ppb) Ni (58) 0.06 0.101 0.078 0.023 Co (59) 0.0008 0.002 0.002 0.000 Ni (60) 0.03 0.102 0.065 0.036 Cu (63) 0.01 0.071 0.069 0.002 Zn (68) 0.08 0.663 0.418 0.245 Mo (95) 0.01 0.047 0.037 0.010 Ag (!07) 0.005 0.004 0.001 0.003 Cd (111) 0.01 0.016 0.023 Ba (138) 0.004 0.001 0.017 Ta (181) 0.007 0.005 0.105 W (182) 0.007 0.016 0.009 0.007 Pb (208) 0.004 0.009 0.005 0.004 20
Spike Recoveries Element Mode Conc (ppb) 1 ppb Spike % Recovered Li (7) Be (9) B (10) Na (23) Mg (24) Al (27) K (39) Ca (40) Ti (47) V (51) Cr (52) Mn (55) Fe (56) 21 0.003 0.002 0.021 0.352 0.088 0.000 0.233 0.115 0.000 0.106 0.084 0.000 0.272 0.940 0.967 0.935 1.494 1.154 1.117 1.404 1.308 0.911 1.110 1.276 1.115 1.192 94 97 91 114 107 112 117 119 91 100 119 112 112 External calibration + internal standard In (cool) & Rh (normal) *Photoresist sample was diluted 1:10 with PGMEA
Spike Recoveries (2) Element Mode Conc (ppb) 1 ppb Spike % Recovered Ni (58) 0.026 1.190 116 Co (59) 0.000 1.135 114 Ni (60) 0.034 1.183 115 Cu (63) 0.009 1.121 111 Zn (68) 0.173 1.248 107 Mo (95) 0.011 0.990 98 Ag (!07) 0.000 0.988 99 Cd (111) 0.000 1.018 102 Ba (138) 0.000 0.981 98 Ta (181) 0.000 1.028 103 W (182) 0.000 1.031 103 Pb (208) 0.000 1.003 100 22 External calibration + internal standard In (cool) & Rh (normal) *Photoresist sample was diluted 1:10 with PGMEA
3 % Photoresist Analysis: Plasma 1.5 min Uptake, 2.5 min Analysis, 5 min rinse with PGME Concentation (ppb) 3.5 3 2.5 2 1.5 1 0.5 0 0 20 40 60 80 100 120 Time (min) 7 Li (3.0%) 24 Mg (0.9%) 25 Mg (1.0%) 27 Al (0.7%) 39 K (0.9%) 40 Ca (2.3%) 52 Cr (1.0%) 53 Cr (0.9%) 55 Mn (0.6%) 56 Fe (3.0%) 58 Ni (1.3%) 59 Co (1.0%) 60 Ni (1.3%) 63 Cu (1.3%) 65 Cu (1.9%) 68 Zn (6.7%) 208 Pb (1.7%) 23 Excellent long-term stability, despite very complex matrix
3 % Photoresist Analysis: Plasma 1.5 min Uptake, 2.5 min Analysis, 5 min rinse with PGME Concentration (ppb) 2.5 2 1.5 1 10 B (2.3%) 47 Ti (1.6%) 51 V (0.9%) 66 Zn (1.4%) 95 Mo (0.8%) 111 Cd (1.0%) 181 Ta (0.7%) 182 W (0.7%) 0.5 0 0 20 40 60 80 100 120 Time (min) 24
7500s Operating Parameters: Optimized for and Plasma ICP-MS Torch Nebulizer Agilent 7500s ShieldTorch System Quartz, 1.5 mm tapered injector Concentric Nebulizer (Self aspiration) RF Power Sampling depth Carrier gas flow rate Make up gas flow rate Optional gas flow rate Spray chamber temp Integration time / mass Plasma 1450W 14.5 1.0 L/min 0 L/min 20% 2 deg C 0.99 sec Plasma 850W 17.5 1.0 L/min 0.37 L/min 20% 2 deg C 0.99 sec 25
Analysis of Photoresist Sample Element Li (7) Be (9) B (10) Na (23) Mg (24) Al (27) K (39) Ca (40) Ti (47) V (51) Cr (52) Mn (55) Fe (56) 26 Mode Detection *Photoresist Limit (ppb) (A) (ppb) 0.0004 0.066 0.002 0.018 0.08 0.922 0.01 0.487 0.005 0.179 0.06 0.186 0.009 0.337 0.02 0.370 0.2 1.809 0.057 0.347 0.057 2.049 0.006 0.066 0.3 1.670 = Not Detected PGMEA (B) (ppb) 0.060 0.025 0.874 0.108 0.086 0.200 0.102 0.250 2.068 0.229 2.154 0.080 1.449 A - B 0.006 0.048 0.379 0.093 0.235 0.120 0.119 0.221
Analysis of Photoresist Sample (2) Element Mode Detection *Photoresist PGMEA A - B Limit (ppb) (A) (ppb) (B) (ppb) Ni (58) 0.06 0.153 0.151 0.002 Co (59) 0.0008 0.075 0.081 Ni (60) 0.03 0.163 0.138 0.025 Cu (63) 0.01 0.132 0.130 0.002 Zn (68) 0.08 0.749 0.516 0.233 Mo (95) 0.01 0.060 0.060 0.000 Ag (!07) 0.005 0.028 0.033 Cd (111) 0.01 0.057 0.062 Ba (138) 0.004 0.033 0.043 Ta (181) 0.007 0.044 0.118 W (182) 0.007 0.036 0.050 Pb (208) 0.004 0.092 0.077 0.015 No observed different in DL s when operating under optimized conditions DL s are fundamentally limited by the metals impurities in the solvent blank 27
Photoresist Analysis - Summary Reliable photoresist analysis requires several key design considerations: Specially designed sample introduction system including a tapered torch Highly efficient plasma rf generator Flexible gas control including the ability for oxygen addition ShieldTorch System for high power, cool plasma operation Higher power (950W) cool plasma effectively breaks down the heavy resist matrix (sample analysed as 2-3% resins) and eliminates C-based interferences on Mg and Cr, as well as Ar-based interferences on K, Ca and Fe Instrument optimization is quick and easy even when operating in multiple plasma modes The Agilent 7500s has the capability to reproducibly measure the required analytes at the levels required by the industry 28